Early clinical studies have shown that intravenous and oral doses of vitamin C may improve symptoms and prolong survival in terminal cancer patients. More recent double-blind placebo-controlled studies have shown that oral adminstration of vitamin C provides no benefit to cancer patients. Conversely, intravenous vitamin C administration raises plasma concentrations as high as 14 mM/L, and concentrations of 1-5 mM/L have been found to be selectively cytoxic to tumor cells in vitro.